When anybody talks, their voice indicator flickers and I don't hear anything.

Already tried voice_enable 1 and voice_loopback 0 Didn’t help.

Would really appreciate some insight here.

Presuming you’ve already restarted your game - if not, do that. I’d suggest playing around with the following setting:

http://image.havasu.co.uk/X0IW1

I’ve often found after a switch of headphones or playing around with other auditorial settings Garry’s Mod will produce no sound and configuring those settings brings it back.
Good luck.

Didn’t help.

Heres a video of my exact problem

(not mine)